Metal-organic frameworks (MOFs) have emerged as a promising class of functional materials due to their microporosity, high internal surface area, and the ability to tune their structural and physical parameters (Ferey, G. Chem. Soc. Rev. 2008, 37, 191-214). These properties have led to the investigation of their application as materials for gas storage, gas separation, and catalysis (Murray, L. et al., Chem. Soc. Rev. 2009, 38, 1294-1314; Farha, O. K. et al., Nat. Chem. 2010, 2, 944-948; Furukawa, H. et al., Science 2010, 329, 424-428; Li, J. R. et al., Chem. Soc. Rev. 2009, 38, 1477-1504; Lee, J. et al., Chem. Soc. Rev. 2009, 38, 1450-1459). In contrast to the numerous reports regarding the application of MOFs toward these goals, there are far fewer reports on strategies for purifying these materials or for controlling their catenation, i.e. network interweaving or interpenetration (Farha, O. K. et al., J. Am. Chem. Soc. 2008, 130, 8598-8599). Nevertheless, a few strategies for regulating MOF catenation have been investigated, including “liquid-phase epitaxy”, solvent or additive templating, solvent and/or concentration manipulation, and rational ligand design (Shekhah, O. et al., J. Am. Chem. Soc. 2007, 129, 15118-15119; Shekhah, O. et al., Nat. Mater. 2009, 8, 481-484; Ma, S. Q. et al., J. Am. Chem. Soc. 2007, 129, 1858-1859; Ma, L. Q. et al., J. Am. Chem. Soc. 2008, 130, 13834-13835; Wang, Q. et al., Inorg. Chem. 2009, 48, 287-295; Song, F. J. et al., J. Am. Chem. Soc. 2010, 132, 15390-15398; He, H. Y. et al., Inorg. Chem. 2010, 49, 7605-7607; Eddaoudi, M. et al., Science 2002, 295, 469-472; Zhang, J. J. et al., J. Am. Chem. Soc. 2009, 131, 17040-17041).
The most widely reported means of controlling catenation is by either solvent or additive-directed templating. For example, Zhou and co-workers have used oxalic acid as a templating agent and 1,10-phenanthroline as a sterically demanding group occupying coordination sites usually reserved for solvent. In a related report, Su and co-workers were able to demonstrate catenation control by guest inclusion in Cd(II)/Mn(II) 2D networks. Lin and co-workers have exploited the steric parameters of their solvent-dimethylformamide (DMF) vs diethylformamide (DEF) to achieve catenation control. A different approach was taken by Zhang et al. and by Eddaoudi et al., who employed low concentrations along with temperature parameters to modulate interpenetration. These strategies constitute important advances, but it remains to be seen if there is broad generality across different linkers, metals, and topologies.
Recently disclosed was an orthogonal approach to influence catenation by ligand design. Catenation can be influenced by modulating the size of substituents projected into the void space of certain MOF materials. This strategy has been successful across different strut types and even when incorporating large tetracarboxylate ligands (Farha, O. K. et al., J. Am. Chem. Soc. 2010, 132, 950-952; Farha, O. K. et al., Inorg. Chem. 2008, 47, 10223-10225). Also investigated was incorporating azolium salts, N-heterocyclic carbine (NHC) precursors, into metal-organic frameworks, a goal that has attracted considerable interest (Lee, J. Y. et al., Inorg. Chem. 2009, 48, 9971-9973; Fei, Z. F. et al., Inorg. Chem. 2005, 44, 5200-5202; Fei, Z. F. et al., Inorg. Chem. 2006, 45, 6331-6337; Chun, J. et al., Inorg. Chem. 2009, 48, 6353-6355; Han, L. J. et al., Inorg. Chem. 2009, 48, 786-788; Chun, J. et al., Organometallics 2010, 29, 1518-1521; Oisaki, K. et al., J. Am. Chem. Soc. 2010, 132, 9262-9264; Crees, R. S. et al., Inorg. Chem. 2010, 49, 1712-1719. While NHCs are versatile ligands for transition metals as well as organocatalysts in their own right, the potential application of coordination polymers containing these heterocyclic motifs is significant (Herrmann, W. A., Angew. Chem., Int. Ed. 2002, 41, 1290-1309; Nolan, S. P. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH: Weinheim Chichester, 2006; Enders, D. et al., Chem. Rev. 2007, 107, 5606-5655. (b) Nair, V. et al., Chem. Soc. Rev. 2008, 37, 2691-2698; Phillips, E. M. et al., Aldrichim. Acta 2009, 43, 55-66; Phillips, E. M. et al., J. Am. Chem. Soc. 2010, 132, 13179-13181; Cohen, D. T. et al., Org. Lett. 2011, 13, 1068-1071; Cohen, D. T. et al., Angew. Chem., Int. Ed. 2011, 50, 1678-1682). Regarding their function as ligands, MOFs bearing NHCs could be functionalized with a metal of choice post-synthetically, yielding reusable heterogeneous transition metal catalysts with permanent microporosity. With respect to organocatalysis, NHCs immobilized in a MOF would not physically be capable of dimerization, a known nonproductive pathway under homogeneous conditions (Arduengo, A. J., Acc. Chem. Res. 1999, 32, 913-921). Heterogeneous materials for catalysis bearing azolium salts have been reported, but these materials lack defined, rigid structure and/or suffer from low porosity (Yadav, J. S. et al., Tetrahedron Lett. 2003, 44, 8959-8962; Barrett, A. G. M. et al., Org. Lett. 2004, 6, 3377-3380; Tan, M. X. et al., Synth. Catal. 2009, 351, 1390-1394; Rose, M. et al., Chem. Commun. 2011, 47, 4814-4816).
It is therefore desired to develop robust systems and increased turnover numbers with suitable azolium-MOF materials. It is further desired to (1) synthesize azolium salts capable of being incorporated into MOFs, (2) incorporate these unique, charged ligands into MOFs, and (3) utilize these metal-azolium frameworks as precursors for catalysts. Herein are reported new metal-azolium framework (MAF) materials using struts that vary the number, size, and electrostatic charge of the “side arm” type functional groups. This approach in turn has led to a new tactic to control catentation or morphology.